Tunnelling in weak ground and under high overburden always proves to be challenging during the design and construction phase. Currently, tunnels are constructed with great lengths, which increases the probability of crossing tectonic fault zones under high overburden. These geological conditions are commonly associated with high loads and massive deformation of the lining. The idea of using ductile elements in the lining has become a well-known practice in such conditions. During the last century, different solutions have been developed. In this paper, the focus is on Telescope-Yielding-Elements (TSR). The big advantage of these elements is that force-oscillation can be reduced to an insignificantly low level and the deformation behavior can be controlled by the use of porous fillings, different types of steel pipes and additional free space. We discuss the results and show the scope of application of such elements. In addition, we highlight how minor modifications of the element configuration can look like to suit Telescope-Yielding-Elements to specific project conditions.
Conventional linings have a tendency to suffer considerable damage in zones of poor rock mass conditions (e.g., tectonic fault zones) with high overburden, which subsequently leads to significant repair costs. To protect the lining from damage, the idea of using highly ductile elements in the lining has become a well-known practice in above-mentioned conditions. During the last decades, different solutions (e.g. yielding elements like the Lining Stress Controller (Moritz 1999), the WABE system (Eisenhütte Bochum) or the hiDCon system (Solexperts)) have been developed to mitigate the adverse effect these high deformations are prone to have on the shotcrete lining (Radončić et al. 2009).
Maximum displacement rates usually occur close behind the tunnel face, where the young shotcrete still has a low strength. Therefore, a yielding element with a deformation controlled stiffness behavior is required to protect the young shotcrete. Energy subjected to the lining and caused by deformation of the ground is transformed to deformation of the yielding elements. With increasing stiffness of the shotcrete lining, the stiffness of the yielding elements should increase as well. The ideal result would be a characteristic yielding-element curve that follows the characteristic time-dependent curve of the shotcrete capacity with respect to the advance rate and the stress regime, prevailing on site.